Well, they 'die' in the sense that they are intelligent beings that get destroyed and you never face those particular models again. Said original models clearly counted as people, and at least Yaridovich and the Axem Rangers knew each other. Yaridovich's mind-read quote is "My promotion's at stake!", and while disguised as the town elder he says something to the extent of having gotten his potion by pleasing his superiors. This, at least to me, implies that he had been active for some time, if he was rising through the ranks - perhaps Smithy decides who gets mass-produced based on the original models' performance in the field.
The mass-produced Machine Mades, however, are weaker and from what I can tell less intelligent - you can read their minds, but they don't really speak to you in the same way the originals did. The Axem Machine Mades in particular move pretty oddly and robotically on the overworld, in a striking contrast to the way the originals moved when you chased them up through the volcano.
Something else that might be worth noting: it's implied that the Factory, or at least the portion of it that we get to see, was only very recently constructed, so perhaps the Smithy Gang previously operated with more unique members in smaller quantities, and only began to shift focus to mass-production of the army once they hit such heavy resistance from the Mushroom Kingdom, which they might not have faced from other worlds.
This is sort of in reference to the post you were quoting, but I've got things to say about this post too, so stick with me: SMRPG doesn't make it entirely clear in narrative, but the player can pick up on the fact (especially after multiple playthroughs) that some bosses are not associated with Smithy at all, and instead have their own goals within the world. Valentina, for instance, somehow managed to usurp the throne in Nimbus Land, but wasn't really working for Smithy at all. Immediately after that story hook, Czar Dragon is basically just a monster who lives in a volcano. The star pieces landed in various locations and many of the bosses aren't even aware of their importance - in fact, Smithy's men aren't even aware until after Bowyer's arrows run back and
tell him about it.
When you enter the factory and see that certain enemies are being mass produced is when you figure out that Smithy only has a handful of members in his "gang": Mack and his shy guy-like underlings, Bowyer and his arrow guys, Yaridovich who is actually 5 guys in a trenchcoat, and the Axem Rangers, along with extremely high ranking members like Boomer, Exor, Domino and Cloaker, Count Down and the factory leads (which probably were never intended to be in combat), and it's probably safe to assume all of these guys are "weapons" - robots built for battle (if you'll recall, Smithy wants to fill the world with weapons").
I always thought of the Machine Made copies as on the way to become new versions of the original - you're just catching them in the middle of production and they literally haven't even had the chance to paint the guys yet. If I had to make a fan-theory about the nature of these enemies, since they seem to be extra-dimensional conquerors, Smithy likely just brings completed versions of the weapons that served him best in whatever world they managed to overrun last, and then sets to making more as soon as he gets his factory in working order. Something like Gunyolk feels like a new prototype for a better weapon, as we never see it being mass produced, and they have it basically locked away in the deepest part of the factory with the chief himself working on it. Mario and friends basically make a tear through the world and Smithy has likely never had this kind of opposition mounted against him, especially not so quickly, so it makes sense when the bad guys tell Smithy that they
just got the place put together and hadn't really finished up the foundation.
As for all of the Smithy heads and who made Smithy, I have one of two theories about that, but both are that Smithy just
exists and has no creator of importance. He seems to be a machine of some sort, and has additional heads in his second form, so I just imagine he modified himself and all of the additional heads are failed attempts at better battle heads. Alternatively, Smithy reaches a new world and, assuming that its total domination will come in a matter of days or at worst weeks, begins work on a copy of himself. This copy would remain and lead the newly conquered world while the original pounds out a new Exor and travels to dominate another universe, repeating this cycle infinitely. What he stands to gain from this is questionable, but surely he simply wants every possible universe to be filled to capacity with weapons.